[PD-ot] Re: [PD] osc

IOhannes m zmoelnig zmoelnig at iem.at
Mon Jan 29 12:28:32 CET 2007


hi.
i moved this explanation to pd-ot.

europa989 at aol.com wrote:
> hi, perhaps the names of these are different than what i'm calling them
> but i was under the impression that a computer has two ip address (at
> least). one is private or local, the other is used by the internet like
> websites. the second can also be found by going to websites like
> http://whatsmyip.org/ .

that is basically all wrong :-)

an IP-address is "something" (a tuple of numbers), that helps to
identify your computer in a network.

a computer might have 2 ip addresses, i have computers which have none
(ok, that's academic as this computer is not networked), 1 (one) or up
to 8 (eight) different ip addresses.
(note: the host with 8 addresses is just the machine with the most
IP-addresses that i administer; it doesn't mean that this is a boundary;
you can have a computer with zillions of IP-addresses).

otoh, several computers can _appear_ to share one IP address.

on the level of addresses, there exists no such concept as "local" or
"private" (what's the difference between 192.168.170.12 and
193.168.170.12??)

since IP-addresses is about communication (possibly international),
there has to be a standardising organisation which takes care of
assigning addresses to "persons", in order to avoid quarrels if people
claim the same IP-address.

however, the standardisers have soon found out, that it is good to have
special addresses, which can be used by everybody (e.g. it would be a
pain if everytime you setup a computer you would have to ask your ISP
for a network address, even if you don't plan to access the so called
"internet")
therefore there are special "reserved" addresses, like "127.0.0.x"
(choose 0<x<255) which is always the computer you are sitting in front
(it is a way to say "I" in IPv4).
another example of such a reserved address is "192.168.x.y", which is
meant to be used in small private networks (a kind of neighbourhood)

you can only communicate with a "private" IP-address, if you are in the
same private network. you cannot access my computer (which has an
IP-address of 192.168.7.141) from where you are sitting now.

.... ( i am getting tired now, so i skip some stuff ;) ....

what you see when you go to http://whatsmyip.org/ might be your
IP-address, or it might as well be not: if your computer is within a
private network, then the address you see is _not_ yours, but it is the
address of a router that handles your (outgoing) traffic.
the router might handle the outgoing traffic for your entire private
network, meaning that all computers in your private network will show
the same ip-address on http://whatsmyip.org/.

if it is not your ip-address, then you cannot send data (including OSC,
but not limited to it) to your computer from the big world outside
without special measures.
in most cases you will be happy that this is so (you wouldn't want me to
hack your computer).
if you have a need to do so, speak to your local administrator or become
one.
the keywords are "firewall" and "port forwarding".

apart from that, there are several worthy readings on the internet which
should explain the internet to you far better than i am able to.




mfg.asdr.
IOhannes



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